The present invention relates to a gas detecting sensor for detecting the partial pressure of oxygen gas contained within exhaust gases discharged from an internal combustion engine and measuring air/fuel ratio of the combustion mixture to be supplied into the internal combustion engine.
Recently, "lean burn system" that is, the system of operating an internal combustion engine with an air/fuel ratio larger than the stoichiometric value thereof, is proposed and employed in practice in order to reduce harmful components contained within the exhaust gases and lower the fuel consumption.
The above described "lean burn system" requires a detecting means for accurately detecting air/fuel ratio in a range of lean mixtures.
U.S. Pat. Nos. 3,933,028 and 4,012,709 show examples of such a detecting sensor as described above.
These detecting sensors are provided with a sensing element made of cobalt monoxide (CoO) or an alloy of cobalt monoxide (CoO) and magnesium oxide (MgO). These detecting sensors are further provided with a heating means for heating and maintaining the sensing element at a predetermined temperature, for example 900.degree. C. in order to prevent cobalt monoxide from changing into tricobalt tetroxide (Co.sub.3 O.sub.4) and to compensate output fluctuation caused by the temperature change.
These conventional detecting sensors have problems as follows.
Since the heating coil is used for heating the sensing element, the structure of the sensor becomes complex and it is troublesome to mount the sensor to the exhaust pipe.
Since the sensing element is formed into a cylindrical or disc-shaped block and the heating coil is arranged around the housing which supports the sensing element, the device becomes large.
Since the sensing element is thick, the sensing element does not exhibit excellent responsive characteristic to the change of the partial pressure of oxygen gas.
Since the sensing element which is supported within the housing, is not directly contacted with the heating coil which is arranged around the housing, high heating efficiency is not obtained so that it takes a while for heating the sensing element to a predetermined temperature and that the consumption of electric power becomes large.
Accordingly, one object of the present invention is to provide a compact gas detecting sensor having simple construction of which sensing element exhibits excellent responsive characteristic.
Another object of the present invention is to provide an improved gas detecting sensor which can be produced with high working efficiency.